The invention relates to an X-ray rotating anode plate and to a method for the production thereof, wherein the X-ray rotating anode plate comprises a base. The base, which carries a layer applied thereon, or a body inserted therein, is made of X-ray active material having a focal path, such as a tungsten-rhenium alloy comprising 5 to 10 mass % of rhenium, and provides the overall design with stability and ability to dissipate heat energy developed during the energetic conversion of electron radiation into X-ray radiation. Important factors for the material of the base in terms of dissipating thermal energy are characteristic properties such as heat capacity, heat conductivity, or heat transfer, and adapting the thermal expansions between, or of, the X-ray active material and the base.
The demands in terms of thermal and mechanical resilience for X-ray rotating anode plates are steadily on the rise. Presently, temperatures of more than 3000° C. may develop at the electronic focal point in the case of high-performance X-ray tubes. In order to improve energy distribution, the plate is rotated at 9,000 rpm. Rotational speeds of 15,000 rpm and higher are planned for the future. For the same reasons, the diameters of rotating anode plates are presently as large as 200 mm, with 300 mm planned for the future. The stability of the base material must be able to meet these requirements.
X-ray rotating anode plates having a base body made of a molybdenum alloy, such as molybdenum with additives of titanium, zirconium, and carbon (“TZM”), have been known for quite some time, for example, from the disclosure of DE 33 03 529 A1. At high rotational speeds of the rotating anode plate, problems are caused by the high density of 10.2 g/cm3 of the primary constituent in the base, which is molybdenum. Such X-ray rotating anode plates can achieve a mass of more than 5 kg. Specifically in the case of computer tomography scanners, the rotation of the X-ray rotating anode plate in the X-ray tube is combined with the rotation and translation of the entire system in which the X-ray tube is located, thereby producing uncontrolled centrifugal forces in several directions. In addition, the immense material costs for the metal designs described above should not be underestimated.
At any given heat capacity, the density, and therefore the mass, of graphite is lower, which is why joined X-ray rotating anode plates, which have a base made of graphite, are known (DE 32 38 352 A1). The stability of graphite at high rotational speeds is entirely insufficient due to the layered microstructure thereof. This is also true of main bodies made of graphite, which have been provided with an X-ray active layer made of tungsten-rhenium by way of vacuum plasma spraying.
Because the stability of the base, based on both molybdenum and graphite, is limited, under the mechanical and thermal loads described above there is a real risk of damage or destruction.
Also, main bodies for these applications that are made of fiber-reinforced graphite are known. Preferably, carbon fibers are used, wherein, for example, adjustment of the thermal expansion coefficient of the base to that of the X-ray active material that is applied (DE 103 01 069 A1), or high thermal expansion in the radial direction, associated with high thermal conductivity in the axial direction (DE 196 50 061 A 1), are achieved by way of the spatial arrangement of the fibers or fiber meshwork. While the carbon fibers mentioned above have good thermal conductivity in the fiber direction, as well as excellent stability, these properties are significantly inferior, by several orders of magnitude, in the direction perpendicular thereto. The last-mentioned technical solution attempted to limit this anisotropy by three-dimensionally interweaving the carbon fibers. However, the material remains anisotropic in the double-digit micrometer range.
A novel carbon-based material consists of so-called carbon nanotubes (CNT), the earliest technical development of which is described in the “Background of the Invention” section European patent DE 695 32 044 T2, which patent is directed to chemically effective functional layers on carbon nanotubes and hence to an entirely different subject matter than that of the present invention.
With conventional graphite, carbon atoms are arranged extending in a hexagonal configuration in individual planes. With carbon nanotubes, such hexagonal arrangements are closed in a tube-like manner, resulting in outstanding mechanical, electrical, and thermal properties. As the prefix “nano” indicates, the diameters of these carbon nanotubes are in the nanometer range. Depending on the source, this means 0.4 nm to 50 nm or 100 nm.
Depending on the manufacturer's information, the bulk density of carbon nanotubes is around 0.15 g/cm3, the material density is given as 1.3 g/cm3 to 1.4 g/cm3, which is clearly below that of graphite. A theoretical value of 45 GPa is given for stability, which would be approximately 20 times that of steel and 200 times that of TZM, the base material mentioned above. The theoretical thermal conductivity is 6000 W/mK, which is double that of diamond materials, and is greater than that of metallic heat conductors by at least one order of magnitude. Furthermore, the use of carbon nanotubes in connection with X-ray tubes is known. These are usually carbon nanotubes in a strictly parallel orientation.
For example, a cathode is known for an X-ray tube, wherein, in order to achieve a cathode surface having small dimensions, the carbon nanotubes are disposed on a plate having negative potential and serve as emitters that emit electrons to an opposing target made of copper (Japanese abstract 2005166565).
In another cathode for X-ray tubes, the nanotubes are disposed behind a control grid and serve to implement a cathode having an adjustable emission surface (Japanese abstract 2006086001).
In addition, a technical solution has been disclosed in which a “forest” or a “floccus” of vertical, parallel carbon fibers having good heat conductivity is arranged on an X-ray active layer (that is, on the electron impingement surface) of X-ray anodes, but carbon fibers employed in this solution are not expressly carbon nanotubes (U.S. Pat. No. 5,943,389). The purpose of this arrangement is to dissipate heat through the carbon fibers, in addition to the heat dissipation through the base.
Furthermore, an X-ray anode is known, on the anode impingement surface of which, carbon nanotubes, preferably in the form of a woven fabric, are disposed in order to suppress the formation of secondary electrons and the development of a plasma and/or the release of neutral gases (WO 03/043036 A1).
Main bodies for X-ray rotating anode are also known in which carbon fibers, preferably carbon nanotubes made of copper (DE 102005039187) or titanium (DE 102005039188), are embedded in order to improve heat dissipation. Copper is disadvantageous in that the melting point thereof is too low for high heat dissipation performance; titanium and copper both have the disadvantage of tending to form carbides at the temperatures at which the carbon is used.
In addition, carbon nanoparticles having a graphite structure, a substantially spherical shape and an average particle size of 55 nm, for example, have recently been disclosed (company publication from Auer-Remy GmbH, Hamburg, Del., “Nanopowders”, item “C-1249YD 7440-44-0”). In addition to the advantageous properties of the carbon nanoparticles in the present context, from a process engineering point of view, when processing the raw materials for shaping the base, achieving a spatial distribution that ensures substantially isotropic properties in the base is naturally less difficult with spherical particles, having the same dimensions in all axial directions, than with carbon nanotubes having an axial extension.
Several of the carbides and nitrides which serve to increase stability in the present invention have already been used for X-ray rotating anodes, but with an entirely different function, and without information as to particle size.
In addition to other compounds, carbides and nitrides of tantalum, niobium, molybdenum, and tungsten have been used for erosion-resistant, liquid metal-lubricated tribological pairings between the rotating anode shaft and the bearing thereof (DE 69 121 504 T2).
In addition to other compounds, tantalum carbide has been proposed for coating the back of the rotating anode plate in order to improve heat emission (DE 2 805 154).
Finally, in addition to other compounds, molybdenum carbide and tungsten carbide are known in arrangements having a plurality of layers for adjusting the thermal expansion coefficient between the X-ray active layer and the base (DE 10 2005 015 920).